Should I write, "I am sorry to get back to you but......" or "I am sorry for getting back to you but....."?
It's an odd sentiment to apologize for.
Do you mean...
"I am sorry for NOT getting back to you but....."? People would apologize for not replying to another person's text or email or phone call (never vice versa).
If this is the case? The latter works with NOT. The former sentence does not work in the negative.